1. Field of the invention
The present Invention relates to an article or person location finding system and method.
2. Description of the Related Art
Many methods of locating articles and persons are known. A relatively coarse location method making use of the cellular telephone system is to determine the location of the base station through which a call is being handled. This will give an indication of the cell in which the caller is located. Typically a cell may be say 20 km across in a lower frequency operating system and significantly less in a micro-cellular system operating at low power at a higher frequency of the order of 1.8 or 1.9 GHz. If a GPS satellite receiver is integrated with a cellular telephone, position can be determined to an accuracy of about 100 m.
WO-A-97 33 386 discloses a location detecting system in which the location of a cellular telephone terminal can be found by the terminal transmitting base station identifications together with electric field strengths of the received radio waves of a plurality of base stations to a position management station which determines the location of the terminal by using the relationship between the electric field strength and the distance between the transmitting points and the receiving point.
A disadvantage of these Known systems is that they require the user to be carrying and using a relatively expensive terminal unit. This makes the system expensive to implement for article tracing or for applications such as automatically tracking or finding children who would not be entrusted with a cellular telephone terminal.
An object of the present invention is to provide a location system having a relatively inexpensive transponding unit.
According to one aspect of the present invention there is provided a radio system comprising a plurality of clusters of spatially separate radio units having transceiving means and received signal strength determining means, each of the radio units having an individual identity, each cluster being associated with an interrogating station comprising transceiving means for communicating with at least the radio units in its cluster, a central station having transceiving means for communicating with a plurality of the interrogating stations and storage means for storing a database comprising the locations of the radio units, and at least one transponding station having transceiving means and its own radio identity, whereby when it is required to determine the location of the transponding station the central station transmits an enquiry signal including its radio identity to the interrogating stations which rebroadcast the enquiry signal, the transponding station in response to hearing its radio identity in an enquiry signal transmits a reply signal including its own radio identity, the radio units in response to identifying the transponding station determine the received signal strength, and each of the radio units relaying the radio identity and the determined received signal strength together with its own identity to at least one of the interrogating stations which relays the information to the central station which computes the location of the transponding station relative to the positions of the radio units.
According to a second aspect of the present invention there is provided a method of locating at least one transponding station having its own radio identity in the radio coverage area of a radio system comprising a plurality of clusters of spatially separate radio units having transceiving means and received signal strength determining means, each of the radio units having an individual identity, each cluster being associated with an interrogating station comprising transceiving means for communicating with at least the radio units in its cluster, and a central station having transceiving means for communicating with a plurality of the interrogating stations and storage means for storing a database comprising the locations of the radio units, wherein when it is required to determine the location of the transponding station, the central station transmits an enquiry signal to the interrogating stations which in turn broadcast the enquiry signal, the transponding station in response to hearing its radio identity in an enquiry signal transmits a reply signal including its own radio identity, the in-range radio units identifying the transponding station and determining the received signal strength and relaying the radio identity of the transponding station, the determined received signal strength and its own identity to the interrogating station which relays the information to the central station which computes the location of the transponding station relative to the positions of the radio units.
According to a third aspect of the present invention there is provided a transponding unit comprising non-volatile storage means for storing the unit""s radio identity, a radio receiver for receiving an enquiry signal, means for comparing the stored radio identity with a radio identity in the enquiry signal, and means responsive to the radio identities being considered to be substantially the same for activating a transmitter to send a reply signal having at least one characteristic different from the received enquiry.
The at least one characteristic may be the frequency of the reply signal being different from that of the enquiry signal or the signalling rate of the reply signal being different from that of the enquiry signal.
In implementing the system, the infrastructure may be a modified version of an installed wide area automatic metering system for use in measuring consumption of for example water, gas, electricity and/or heating water in domestic and business premises. Thus in urban areas the radio unit comprises the telecommunications part of a metering unit coupled to each of the premises which will enable a resolution equivalent to one dwelling to be obtained. The network interrogating stations are normally mounted in advantageous positions, for example on posts, from a radio communications point of view. In rural areas where the radio units may be dispersed less densely than in urban areas, the networK interrogating stations may function as radio units for the purpose of location finding and information is relayed to the central station by way of one or more intermediate network interrogation stations functioning as relay stations.
A transponding station may be physically small so that it can comprise an accessory which can be attached to clothing, especially children""s clothing, or integrated into a container for an article or attached to the article. The transponder station is typically a transceiver which is able to receive an enquiry signal at one frequency and transmit a signal at another frequency.